Expression (2)Program Design (I)

2021 Fall Fu-Yin Cherng

Dept. CSIE, National Chung Cheng University

Outline

● Increment and Decrement Operators● Expression Evaluation

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So far, what is the level of difficulty of this course for you?

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Have you discussed anything (e.g., program exercise, homework) with your teammates?

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Do you finish and upload homework 1 and 2?

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Increment and Decrement Operators

● Two of the most common operations on a variable are “incrementing” (adding 1) and “decrementing” (subtracting 1)

● Incrementing and decrementing can be done using the compound assignment operators

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i = i + 1;

j = j - 1;

i += 1;

j -= 1;

Increment and Decrement Operators

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● C provides special ++ (increment) and -- (decrement) operators.● The ++ operator adds 1 to its operand. The -- operator subtracts 1.

○ can be used as prefix or postfix operators .● side effects: they modify the values of their operands.

○ i + 1; (this expression didn’t change the value of i; since + didn’t change value of the operand)

int i = 1, j = 1;

++i; // prefix; i value is changed to 2

i++; // postfix; i value is 3

--j; // prefix; i value is 0

j--; // postfix; i value is -1

Increment and Decrement Operators

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● ++i: pre-increment (prefix increment)● i++: post-increment (postfix increment)

○ produce the result of i, but increment i by 1 afterwards

i = 1;

printf("i is %d\n", ++i); /* prints "i is 2" */

printf("i is %d\n", i); /* prints "i is 2" */

i = 1;

printf("i is %d\n", i++); /* prints "i is 1" add i by 1 afterward */

printf("i is %d\n", i); /* prints "i is 2" */

Increment and Decrement Operators

● ++i means “increment i immediately” ● i++ means “use the old value of i for now, but increment i later.”

○ How much later? ○ The C standard doesn’t specify a precise time, ○ but it’s safe to assume that i will be incremented before the next statement is executed.

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Increment and Decrement Operators

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● postfix -- and ++ ○ have higher precedence than unary plus and minus○ left associative

● prefix -- and ++○ have the same precedence as unary plus and minus○ right associative

int i = 1;

+i--++; // +((i--)++)); i is 1

i = 1;

----+i; // (--(--(+i))); i is -1

However, don’t recommend write program like this way! Hard to understand.

Increment and Decrement Operators

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● Does ++ and -- work with float variable?○ Yes, the increment and decrement operations can be applied to

floating-point numbers. ○ However, it’s rare to increment or decrement a float variabel

Expression Evaluation

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Precedence Name Symbol Associativity

1 (high) increment (postfix) variable++ left

decrement (postfix) variable--

2 increment (prefix) ++variable right

decrement (prefix) --variable

unary plus +variable

unary minus -variable

3 multiplicative * / % left

4 additive + - left

5 (low) (compound) assignment = *= /= %= += -= right

Expression Evaluation

● The table can be used to add parentheses to an expression that lacks them.○ just look up the table when you need it

● However, certainly don’t recommend write this kind of statement. ● Just help you to interpretate correctly if you encounter in the future

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a = b += c++ - d + --e / -f;

(a = (b += (((c++) - d) + ((--e) / (-f)))));

Order of Subexpression Evaluation

● The value of an expression may depend on the order in which its subexpressions are evaluated/executed.

● C doesn’t define the order in which subexpressions are evaluated○ with the exception of subexpressions involving the logical and, logical or, conditional, and

comma operators (introduce in later chapters)● For example,

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(a + b) * (c - d)

//don’t know whether (a + b) will be evaluated before (c - d).

Order of Subexpression Evaluation

● Most expressions have the same value regardless of the order in which their subexpressions are evaluated.

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int i = 0, j = 1;

i = (1+j) * (2+j); //(a + b) * (c - d)

// the value of i will be 2*3

// no matter (a+b) or (c-d) is evaluated first

Order of Subexpression Evaluation

● However, this may not be true when a subexpression modifies one of its operands

● For example, ○ if (b = a + 2) is evaluated first, c will be 6○ if (a = 1) is evaluated first, c will be 2

● Avoid writing this kind of expression!!○ access the value of a variable and also modify the variable in the same expression!

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a = 5;

c = (b = a + 2) - (a = 1);

// warning message “operation on ‘a’ may be undefined”

a = 5;

b = a + 2;

a = 1;

c = b - a;

// c wil always be 6

only use one assignment operator in one statment

Order of Subexpression Evaluation

● Besides the assignment operators (=), the only operators that modify their operands are increment (++) and decrement (--).

● When using these operators, be careful that an expression doesn’t depend on a particular order of evaluation.

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i = 1; // = assign 1 to i, so change value of i

printf("%d\n", ++i); //output: 2

// ++i add 1 to i and update i, so change value of i to 2

Undefined Behavior

● Statements such as c = (b = a + 2) - (a = 1) cause undefined behavior.

● Possible effects of undefined behavior○ The program may behave differently when compiled with different compilers.○ The program may not compile in the first place.○ If it compiles it may not run.○ If it does run, the program may crash, behave erratically, or produce meaningless results.

● Undefined behavior should be avoided.

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Expression Statements

● C has the unusual rule that any expression can be used as a statement.● For example,

○ getting the value stored in i○ incremented the value○ but then discarded○ i’s value is still 0

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int i = 0;

i + 1;

// value of i is 0

Expression Statements

● Since its value is discarded, there’s little point in using an expression as a statement

○ you need a place (e.g., variable) to store the value● unless the expression has a side effect

○ change the value of operand (assignment, increment, and decrement operators)

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i = 1; /* useful; assign 1 to i */

i--; /* useful; add i by 1 and update to i*/

i * j - 1; /* not useful; no variable store result of expression*/

Expression Statements

● A slip of the finger can easily create a “do-nothing” expression statement.● Some compilers can detect meaningless expression statements; you’ll get a

warning such as “statement with no effect.”● This kind of errors is more common than you might expect

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// goal: i = j;

i + j; //accidentally type

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Given int i = 1, what is the value of i after the expression i--; ?

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int i = 1, j = 2, what is the value of i and j after the expression: i * j / i?

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int i = 1, j = 2, what is the value of i and j after the expression: i += j;

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Summary

● Increment and Decrement Operators○ ++ and --○ prefix and postfix

● Expression Evaluation○ Precedence○ Undefined Behavior

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The Programming ProcessProgram Design (I)

2021 Fall Fu-Yin Cherng

Dept. CSIE, National Chung Cheng University

The Programming Process

● All programming involves creating something that solves a problem.● rough guide to the things you should do when entering the land of

programming.

● Identify the Problem● Design a Solution● Write the Program● Check the Solution

27https://www.cs.bham.ac.uk/~rxb/java/intro/2programming.html

actually programming, but probably the least important stage

Identify the Problem

● In the course, this stage is actually identifying the solution○ since we describe problem clearly in homework and program exercise for you○ however, in real situations, you often need to identify the probelm which can be solve by

programming■ For example, how to help people register vaccination?

○ involving computation thinking, design thinking, software desing… will not be covered in this course

28https://www.cs.bham.ac.uk/~rxb/java/intro/2programming.html

Identify the Problem

● Two stages to identifying a solution○ Requirements

■ for example, if you were asked to write a calculator program, what are the requirments?■ entering equations, pressing buttons, printing results on the screen

○ Specification■ decide exactly what your solution should do to fulfil requirements

● usually there are different solutions to a single problem● decide on which of those solutions you want

29https://www.cs.bham.ac.uk/~rxb/java/intro/2programming.html

Design a Solution

● How you're going to turn that specification into a working program○ a program is simply a list of steps describing to the computer what it should do○ design is simply a higher-level description of those steps

● Pick up you pen and paper, draw the blueprints of your program○ for example, you can write pseudocode or flow charts

● When your design is completed you should have a very clear idea of how the computer is going to fulfil your specification

30https://www.cs.bham.ac.uk/~rxb/java/intro/2programming.html

Design a Solution

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Write the Program

● Programming is then the task of describing your design to the computer○ tell the computer your way of solving the problem

● three stages to writing a program○ Coding

■ translating the design into an actual program■ now, you sit down at the computer and type

○ Compiling○ Debugging (essential technique you need to learn as a programmer)

■ your program it is likely to come back to you with a long list of mistakes■ perfectly normal, even for experienced programmers■ So, don’t worry when you see any error or warning messages■ What you need to do is debugging by yourself first

32https://www.cs.bham.ac.uk/~rxb/java/intro/2programming.html

Write the Program - Degugging

● Debugging is simply the task of reading error/warning messages, looking at the original program, identifying the mistakes, correcting the code and recompiling it.

○ This cycle of code -> compile -> debug will often be repeated many many times● it’s not necessary to write the entire program before you start to compile and

debug it!○ In most cases it is better to write a small section of the code first, get that to work, and then

move on to the next stage. ○ This reduces the amount of code that needs to be debugged each time and generally creates

a good feeling of "getting there" as each section is completed.

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Write the Program - Degugging

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compile and debug; ensure the programcan read X and Y successfully.

compile and debug; ensure the programcan add X and Y successfully.

compile and debug; ensure the programcan disply SUM.

Check the Solution

● check whether what you've written actually solves your original problem.○ running it and playing with it for a bit to see if it seems to be working correctly○ example input and output

● check that it meets the requirements and specification● trying to figure out where in the code the mistake is.

○ Once identified, the problem should be fixed by changing the code and recompiling.

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"Another effective debugging technique is to explain your code to someone else. This will often cause you to explain the bug to yourself.

Sometimes it takes no more than a few sentences, followed by an embarrassed "Never mind, I see what's wrong. Sorry to bother you." This works remarkably well; you can even use non-programmers as listeners.

One university computer center kept a teddy bear near the help desk. Students with mysterious bugs were required to explain them to the bear before they could speak to a human counselor."

Brian Kernighan

That’s why we have teams and ask you to describe your problem clearly when writing a email.